Abstract
Keywords: CI engine, fuel injection, solution, indicator graph, emission
The specificity of the combustion process being realized in the direct injection compression-ignition engine (CI) consist in a fact that the liquid fuel in a form of fuel sprays is supplied to the engine combustion chamber right before the piston top dead centre (TDC). Thus a complete process of preparing the mixture for being burnt, i.e. a disintegration of the fuel spray into drops, their evaporation and mixing with air needs to be performed in a very short time. For such a way of fuelling the engine some local and very significant differences of the excess air factor values are met. The values met in the combustion chamber range from the infinite high one in the area which is not covered by the fuel spray, throughout 0.8 ÷ 1.5 at the edges of the fuel spray initializing the ignition to 0 being recorded in the spray core. The local deficiencies of oxygen occur despite the high value of the global excess of air the value of which changes with the engine load and ranges from 11 at the engine idle running to 1.4 ÷ 1.3 at the engine full load, i.e. for the operating conditions corresponding to the outer engine operating characteristic.
In case of the combustion process the local deficiencies of oxygen are one of the most important reasons for forming carbon oxides, hydrocarbons, and partly of forming the particulate matter, whereas the formation of nitrogen oxides is mainly connected with the kinetics of developing the flame which generates the value of the produced heat as the heat delivery speed determines the level of temperature in a combustion chamber. All of it causes that the fuel spraying, in addition to the air swirl, is of the decisive importance for preparing the mixture. The quality of spraying is determined by two basic physical factors: the pressure existing in the nozzle area right before the nozzle whole and the pressure in the combustion chamber where the fuel spray is directed to. An increase in the injection pressure values improves the fuel spraying and it is a current preferred tendency of the developments of the injection systems for the CI engines. The changes which are done in the Common Rail (CR) system confirm the above statement. Every next generation of this system is characterized by the injection pressure values that are higher that ones of the previously generation. The improvement of the fuel spraying can be achieved not only by increasing the injection pressure but also by modifications to the mechanism causing the disintegration of the fuel spray. The velocity of the fuel outflow from the nozzle is a single physical stimulus which causes the disintegration of fuel spray in the currently used mechanism of spraying. In order to achieve the improvement of spraying it is proposed to use in the discussed mechanism an additional physical stimulus resulting from the physical properties of the gas-in liquid solution. The amount of gas that can be dissolved in a liquid significantly depends on the pressure. A spontaneous release of gas at the non-equilibrium state caused by the pressure fuel is very characteristic for such a solution. The process of releasing the gas from a liquid is of a volumetric nature, i.e. the gas is being released simultaneously from the whole liquid volume. The energetic effects which accompany that process depend on the speed of the stimulus modifications and the gas which is released always presents a tendency to break the bonds of liquid molecules. Under such conditions the state of liquid is similar to that state of boiling. The presented properties of a liquid are very desirable in the injection system of the diesel engine. Thus a concept of using the effect accompanying the process of releasing the gas from a liquid for improving the existing mechanism of fuel oil spraying has been developed.